The complex sequence of events which incite electrical instability in the heart leading to sudden cardiac death (SCO) are poorly understood. Consequently, SCO remains a major unresolved public health problem. Although heart failure (HF) clearly predisposes to SCO, the mechanisms linking mechanical to electrophysiological dysfunction in the heart are unclear. Over the past 10 years, the PI's research has produced recognition that beat-to-beat alternation of cardiac repolarization (i.e. alternans), is a highly sensitive marker of susceptibility to SCO. In the absence of alternans, patients with HF exhibit resistance to SCO. Moreover, the PI recently showed that cellular alternans is linked to a novel electrophysiological mechanism of arrhythmogenesis. However, essentially all previous research on alternans has been restricted to normal myocardium, whereas SCO occurs most commonly in patients with HF. Therefore, we propose to move the field of cardiac alternans in a fundamentally new direction. We hypothesized that cellular alternans is an important mechanism linking HF to cardiac arrhythmogenesis. Specifically, HF- induced changes of key calcium cycling proteins enhances susceptibility to cellular alternans, and, in turn, arrhythmias. Our hypothesis is based on recent findings that HF disrupts two major calcium cycling processes;SR calcium release (by several mechanisms including impairment of the FKBP12.6-RyR macromolecular complex and/or the T-tubule network), and SR calcium reuptake (from impaired SERCA2a). We propose an integrative approach using techniques of high-resolution dual voltage-calcium whole heart mapping, subcellular calcium imaging in myocytes, targeted gene expression, and voltage-clamp studies. The aims of this project are to: 1. Determine the role of dysfunctional SR calcium release and reuptake in enhancing susceptibility to alternans in HF, 2. Determine the electrogenic mechanism by which calcium cycling alternans causes alternans of cellular repolarization in HF, 3. Determine the mechanism linking cellular alternans to arrhythmogenesis in HF, and 4. Use targeted gene expression as a strategy for suppressing arrhythmogenic alternans, thereby engineering the "fribrillationless" or "fibrillation-resistant" heart. These studies are expected to improve understanding of the functional organization of electrical activity in HF, and provide fundamentally novel insights into mechanisms and therapies for SCO.